Both P and M pathways contribute to depth perception or stereopsis, which is dependent on slight differences in the retinal images formed in the two eyes. Stereopsis is useful only for relatively nearby objects. However, in such cases, these disparities provide visual cues about depth. Interestingly, the anatomy of the visual pathways indicates that depth perception must be a cortical function because it depends on convergent input from the two eyes and the left/right eye inputs are parallel but segregated in the LGN and in Layer 4 of striate cortex.
The separation of M and P pathways from the retina through the thalamus and all the cortical regions raises the issue of how all the parts are combined to account for the clear, coherent images of events, objects, and persons that humans perceive. It seems unlikely that all the components that represent a percept, such as parts of a face and whether that face belongs to a familiar person, are somehow converged onto a single neuron that recognizes it. Rather, complex percepts probably arise from the coordinated activity of large sets of neurons across multiple regions of the CNS. The process by which a “binding” of such disparate neuronal elements into a percept is unclear, but one working hypothesis is that it may be accomplished by the temporal synchronization of many anatomically distributed neural events.
Lesionsoftheextrastriatevisualcortexcanproducevariousdeficits.Bilaterallesionsoftheinferotemporalcortexcanresultincorticalcolorblindness(achromatopsia) orinaninabilitytorecognizefaces,evenofclosemembersofthefamily(prosopagnosia). AlesioninareaMTorMSTcaninterferewithmotiondetectionandeyemovements.
CHAPTER 8 The Special Senses •Fig. 8.15 Twopuretonesareshownbythesolid anddashed lines. Frequencyisdeterminedfromthewavelengthasindicated.Amplitudeisthepeak-to-peakchangeinsoundpressure.Thetwotoneshavethesamefrequencyandamplitudebutdifferinphase.
The superior colliculus of the midbrain is a layered structure that is important for certain types of eye movements (see ). The three most superficial layers are involved exclusively in visual processing, whereas the deeper layers receive multimodal input from the somatosensory and auditory systems, as well as the visual system, particularly from cortical areas involved in eye movement.
Another retinal projection is to the pretectum, which bilaterally activates parasympathetic preganglionic neurons in the Edinger-Westphal nucleus that cause pupillary constriction in the pupillary light reflex. The pretectal areas are also interconnected through the posterior commissure, and thus the reflex causes both ipsilateral (direct) and contra-lateral (consensual) pupillary constriction when a light is shown in one eye.
The visual pathways also include connections to nuclei that serve functions other than vision. For example, a retinal projection to the suprachiasmatic nucleus of the hypothalamus controls circadian rhythmicity (see
The Auditory and Vestibular Systems
The peripheral parts of the auditory and vestibular systems share components of the bony and membranous labyrinths, use hair cells as mechanical transducers, and transmit information to the CNS through the vestibulocochlear nerve (CN VIII). However, the CNS processing and sensory functions of the auditory and vestibular systems are distinct. The function of the auditory system is to transduce sound. This allows us to recognize environmental cues and to communicate with other organisms. The most complex auditory functions are those involved in language. The function of the vestibular system is to provide the CNS with information related to the position and movements of the head in space. Control of eye movement by the vestibular system is discussed in
Sound is produced by compression and decompression waves in air or in other elastic media, such as water. Sound frequency is measured in cycles per second, or hertz (Hz). Each pure tone results from a sinusoidal wave at a particular frequency and is characterized not only by its frequency but also, instantaneously, by its amplitude and phase (